The invention relates to a process for drying and preheating coal by utilizing the sensible coke heat in the dry cooling or quenching of coke in a three-cycle system, where the coke heat is transferred, in a first gas cycle, directly to a cooling gas (inert gas), and is then indirectly reacted completely to steam, and where the heat energy of this steam passes over, in a second steam cycle, to the coal, and where the coal is fluidized in a third cycle by means of steam or inert gas.
The term "three-cycle system" for such processes, has been generally accepted in the industry. It is known to use the sensible coke heat for generating steam (DAS No. 2,738,442) in the dry cooling of coke.
A coal drying process is also known where the coal particles to be dried are suspended in a fluid bed in a drying chamber, and the coal particles are dried in the chamber by means of heating coils to which steam is supplied, and the steam is heated in the heating coils by a heat exchanger. The heat for the heat exchanger is taken from the coke oven cooling gas which is used in the dry cooling of coke from a coke oven. The gas used for the fluid bed can be heated by the heat exchanger, from which heat is withdrawn from the steam in the heating coils. The gas for the fluid bed is a part of the steam which is heated in the heat exchanger and fed to the heating coils, so that the same steam is used for the fluid bed and for the heating coils (DOS No. 2,342,184).
These methods are most economical when the heat-absorbing and heat-emitting media are circulated in a complete cycle, and single or several gas cycles have already been realized in the known methods. In the method according to DOS No. 2,342,184, the coke heat is first given off in a completely closed inert gas cycle to a gas, and transferred indirectly to water for the preparation of steam. The steam transfers its heat energy in a second cycle by indirect contact with the coal particles to be dried, and the returns to the coke heat transfer device for resuperheating. Steam, which can likewise originate from the coke heat, can be used as a fluidizing medium for the fine coal to be dried, and coked in a drying vessel. This is not done in a cycle, however, which returns to absorb coke heat, but in an open system where the steam is lost. A cycle only takes place over the drying vessel.
In addition, this process is carried out at a temperature at which sulfur compounds are released from the coal to be dried, which could not be left in the circulating steam. Consequently, it is necessary to treat the steam and to condense it. Though the condensate can be treated together with the condensates of the coke oven gas, this requires additional installations for the preliminary coke drying plant which burden the entire system. In addition, desulfurizing agents are adeed to the dried fine coal which must be removed again after drying. This interlocking of preliminary coal drying and desulfurization is economically unfeasible and requires a considerable amount of equipment. Furthermore, the steam which has come in direct contact with the coal cannot be returned into the coal drying plant, and is therefore lost. The process according to DOS No. 2,342,184 is therefore not a three-cycle process, but a two-cycle process. A return of the direct drying steam into the total steam cycle system and reprocessing of the coke heat is not provided.
From DOS No. 2,647,079, it is known to effect the predrying and the preheating of the coal, exclusively in a recycled indirectly superheated steam atmosphere, and to withdraw the portion originating from the wet coal from the steam cycle. The coke quenching heat for predrying is taken from the coal in such a way that the coal predrying gas cycle is combined with the coke quenching gas cycle according to DOS No. 2,415,758 into a single cycle. In the drying method according to DOS No. 2,647,079, considerable quantities of gas or superheated steam must be recycled and separated from the coal by cyclones, since the heat is transferred directly to the coal, exclusively.
Not all gas cycles are closed in all known methods. There are large quantities of gas that have to be treated with the resulting technical and commercial expenditures.